1. Field of the Invention
The subject invention relates, generally, to gerotor pumps, more specifically to a gerotor pump having a housing that allows for variations in its internal axial tolerances.
2. Description of the Related Art
Gerotor pumps are commonly employed in the automotive industry for pumping oil to lubricate and cool various components of a typical automotive power train. For example, pumps of this type are often employed in transfer cases of automotive transmission assemblies. Gerotor pumps typically include a gerotor set having an externally toothed inner gear rotor intermeshed with an internally toothed outer gear rotor, wherein each rotor is disposed about respective eccentric axes. Other rotor pump sets are known that employ smooth surfaced rotors, one disposed within another and placed about respective eccentric axes. Regardless of the structure of the rotor set, the pumping action is accomplished by the rotational opening and closing of voids, or volumes, formed by the offset of the inner to the outer rotor during their rotation within a pump housing.
The gerotor set is typically seated in a cylindrical inner chamber of a pump housing and enclosed by an outer cover plate fixedly secured to the pump housing. A drive shaft rotates the inner rotor against the outer rotor to pump fluid between the intermeshed teeth from an inlet to an outlet in the housing.
The efficiency of the pump depends greatly upon the axial sealing of the gerotor set seated in the inner chamber between the pump housing on one axial side of the gerotor set and the outer cover plate on the other axial side of the gerotor set. The axial sealing of the gerotor set between the pump housing and the cover plate requires close manufacturing tolerances of the axial depth of the inner chamber housing the gerotor set. Such close manufacturing tolerances results in increase labor and expensive machining costs of the pump. The close tolerances also require a high torque input to initiate the rotation and pumping action of the pump elements. Additionally, in operation, when constructed with the necessary close tolerances for efficient pumping, the pump can easily exceed the desired design output pressure requiring supplemental structure for pressure relief.
Lastly, current manufacturing practices often call for the use of different materials for the pump elements and the pump housing to save weight. The use of different materials results in the undesirable effect of variations in the axial clearances as a function of temperature changes within the pump due to differing material coefficients of expansion. These temperature related changes in axial clearances, or tolerances, cause wide variations in pump performance.
Accordingly, it is desirable to provide a gerotor pump having a housing for enclosing the gerotor set which not only reduces the need for the close, exacting tolerances of prior art pumps, but also allows for variations in the thermal expansion of the pump elements while lowering the rotational torque input required to start the pump. Therefore, it is desirable to provide a gerotor pump having a variable tolerance housing.
The deficiencies in the related art are overcome by the present invention in a gerotor pump for pumping pressurized fluid between an inlet port and an outlet port. The gerotor pump includes a pump housing defined by a pump cover and a pump end plate. The pump cover defines an open end, an axial bore, a bottom end, and a central opening in the bottom end. The pump end plate is adapted to matingly engage the open end of the pump cover thereby enclosing the axial bore of the pump cover and creating an inner cylindrical chamber in the pump housing. A wear plate having a central opening is operatively disposed within the inner cylindrical chamber of the pump housing. Also, a gerotor gear set assembly having an inner and an outer rotor is disposed within the inner cylindrical chamber of the pump housing such that the gerotor gear set assembly is operatively set between the wear plate and the pump end plate. A shaft is received through the pump cover and the wear plate through the central openings and is operatively connected to the gerotor gear set assembly for rotating the gerotor gear set assembly within the inner cylindrical chamber of the pump housing. A biasing member is operatively disposed between the bottom end of the pump cover and the wear plate. The biasing member is adapted to bias the wear plate against the gerotor gear set assembly and the pump end to create an axial zero tolerance condition.
The present invention thereby overcomes the disadvantages and drawbacks of the current art by reducing the need for the close, exacting tolerances of prior art pumps, and also allows for variations in the thermal expansion of the pump elements while lowering the rotational torque input required to start the pump. This is accomplished by the variable tolerance characteristics of the pump housing that are available due to the use of the biasing member within the housing that causes the gerotor assembly to be held in zero tolerance to the other pump elements.